The scanning tunneling microscope (STM) can provide high resolution three-dimensional (3D) images of the surface topography of conducting samples. STM has been used extensively to study surface structure at the local atomic level. With its high lateral and vertical resolution, STM can also reveal important topographic details within larger scale structures, which cannot be resolved by conventional measurement techniques..sup.1 For example, the usefulness of STM has been demonstrated for diamond-turned samples,.sup.1,2 ruled grating replicas,.sup.2 x-ray reflecting optics,.sup.3 and optical disks,.sup.4-6. Many of these precision surfaces have features ranging from 10 to 1,000 nm in height and less than 2 .mu.m in width.
Tips for STM have been prepared using several fabrication techniques, the most common of which include electrochemical etching and mechanical shearing. Many electrochemical polishing procedures, solutions, and conditions appropriate for tip specimen preparation in field ion microscopy (FIM) are also applicable for STM tips..sup.7 Tungsten tips, which fulfill the STM instrument requirement of being stiff, have been used to a great extent to image specimens..sup.8 The presence of an aqueous oxide layer, however, makes it difficult to acquire STM images with these tips unless the surface oxide is removed by evaporation in an ultrahigh vacuum (UHV) system just prior to imaging in vacuum, by annealing before imaging in air, or by sputtering in an ion mill..sup.9 Platinum, although a softer metal, is a preferable material to W because it is inert to oxidation. The addition of Ir to form a Pt/Ir alloy adds stiffness while maintaining a chemically inert material. Tips of Pt/Ir have been electrochemically etched in solutions of CaCl.sub.2 /H.sub.2 O/HCl.sup.10 or KCl/ H.sub.2 O/HCl.sup.7, NaCN/KOH,.sup.11 and molten NaNO.sub.3 /NaCl..sup.12
Other STM tip preparation methods have been reported. Reproducible sharp-pointed Mo tips can be prepared by controlling the applied ac voltage, wave shape, phase angle, frequency, and number of waves..sup.13 Ion milling.sup.14 and field ion microscopy (FIM).sup.15 techniques have been used to prepare tips from single crystal W. Tips capable of achieving atomic resolution on graphite include pencil lead,.sup.16 colloidal graphite-coated tungsten,.sup.16 and cut Pt/Ir wire (available from Digital Instruments Inc.). The cut tips, although variable in shape, produce good quality images from samples with atomic level topography, e.g., graphite and thin metal films..sup.17
Although resolution requirements are usually not as stringent for highly topographic samples, wide area scans place unique restrictions on tip morphology. For such samples, symmetric, controlled geometry tips with small radii of curvature and high aspect ratios are necessary to minimize the convolution of the tip shape into the acquired image. In addition, specially shaped tips are desireable for large feature imaging. For example, a tip 500 .ANG. in radius with a 15.degree. cone half angle cannot resolve a square-bottomed groove as well as a tip 500 .ANG. in radius with a 5.degree. cone half angle. On the other hand, tips which have too small a cone half angle might be mechanically fragile and prone to vibration. Comparatively little attention has been devoted to optimizing tip geometry for STM.